The present invention relates to a method and apparatus for printing on sheet material and is particularly useful for printing signs and other artistic designs in accordance with a printing program.
U.S. patent application Ser. No. 08/007,662, filed Jan. 22, 1993 describes, among other things, thermal printing apparatus that can be used to generate signs, designs, characters and other graphic images on a strip of sheet material in accordance with a stored printing program. The program is read and translated into machine commands by a microprocessor-based controller and causes the stored image to be generated on the strip of sheet material by the printer. The printer is preferably a thermal printer having a thermal printhead mounted in stationary relationship with respect to the strip of sheet material during a printing operation, and the strip of sheet material is fed under the printhead by a mechanical drive mechanism. The controller coordinates the operation of the printhead and the mechanical drive mechanism in order to place the printed image at a desired location on the strip of material. In one form, the strip of material is a strip of vinyl secured to a backing material by a pressure sensitive adhesive so that after printing the vinyl bearing a printed image can be cut and stripped from the backing material and thereafter placed on an appropriate sign board.
The thermal printer in the referenced application utilizes a web of thermally releasable inking material to produce images in color. Multi-colored images can be produced by the printer simply by passing the strip of sheet material through the printer relative to the printhead two or more times and substituting a web bearing an inking material of a different color on each pass. When the images are produced in separate passes and, therefore at different times, small misalignments between the images or distortions of just one image may create noticeable errors or defects that detract from or totally destroy the resulting product. Many printed products such as commercial signs or artwork require high quality printing without notable distortion or registration errors. Such errors can arise at the beginning of each printed image simply because of backlash in the drive mechanism that moves the printhead and sheet material relative to one another.
It is accordingly a general object of the present invention to provide a method and apparatus by which the distortion and error associated with backlash are eliminated from images that are prepared by a printing apparatus having a drive mechanism for moving the printhead and sheet material relative to one another during a printing operation.